Organic light emitting diode having electron and hole mobility in light emitting layer and display using the same

ABSTRACT

An organic light emitting diode comprising a pair of electrodes and a stack including a hole transport layer, a light emitting layer, and an electron transport layer, the stack being intermediate between the electrodes, the light emitting layer being of a material having hole mobility and electron mobility equal to or lower than hole mobility of the hole transport layer and electron mobility of the electron transport layer, respectively.

This is a divisional of application Ser. No. 11/362,852 filed Feb. 28,2006. The entire disclosure of the prior application, application Ser.No. 11/362,852, is considered part of the disclosure of the accompanyingdivisional application and is hereby incorporated by reference. Thisapplication claims benefit under 35 U.S.C. §119 from Japanese PatentApplication No. 2005-052911, filed on Feb. 28, 2005, the entire contentof which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to an organic light emitting diode (hereinafterabbreviated as “OLED”) and a display using the same. More particularly,it relates to an OLED with increased luminescence efficiency andextended life and a display using the OLED.

BACKGROUND OF THE INVENTION

OLEDs using an organic substance are promising for applications toinexpensive, large area, full-color, solid-state emissive displays orwriting light source arrays and have been under active studies. OLEDsgenerally have a light emitting layer sandwiched between a pair ofopposing electrodes. With an electric field applied to the electrodes,electrons and positive holes are injected from the cathode and theanode, respectively, into the light emitting layer, where they arerecombined. Whereupon the electrons fall from the conduction band to thevalence band, emitting light as an energy difference between the bands,which is observed as luminescence.

Conventional OLEDs have low luminescence efficiency and low durability,i.e., a short service life. Various techniques providing solutions tothese problems have recently been proposed.

For example, JP-A-2003-68466 discloses a light emitting diode comprisinga substrate having formed thereon an anode, a cathode, and a lightemitting layer intermediate between the electrodes. The light emittinglayer contains a host material and a dopant incorporated in the hostmaterial. Aiming to improve luminescence efficiency and life, the dopantcomprises a light emitting material and a non-light-emitting material.However, the system using BCP (bathocuproine) recited in JP-A-2003-68466is unsatisfactory in life because of the high ionization potential ofBCP (6.1 eV), still leaving room for improvement.

JP-A-2003-77674 discloses a light emitting diode, the light emittinglayer of which contains (1) a host material having electron transportand/or hole transport properties, (2) a first compound that producesphosphorescence at room temperature, and (3) a second compound thatproduces phosphorescence or fluorescence at room temperature and has alonger maximum emission wavelength than the first compound. The secondcompound is allowed to emit light at high efficiency. More specifically,the second compound is either a phosphorescent compound that does notemit light alone at high efficiency or a fluorescent compound that hasvarious emission colors but with not so high emission efficiency as aphosphorescent compound at any wavelength. The disclosure teaches thatthe first compound showing phosphorescence at room temperature, whencombined with the second compound, serves as a sensitizer to enhance theluminescence of the second compound. However, the proposed lightemitting diode is insufficient in life, and an improvement is stillawaited.

SUMMARY OF THE INVENTION

In order to put an OLED into practical use as a large area, full colordisplay, it is necessary to further improve the state-of-the-art OLEDsin luminescence efficiency and durability to achieve a prolonged servicelife.

An object of the present invention is to provide an OLED having highluminescence efficiency and an extended life and to provide a practicaldisplay using the OLED.

To accomplish the above object, the invention provides followingaspects.

(1) An organic light emitting diode comprising a pair of electrodes anda stack including a hole transport layer, a light emitting layer, and anelectron transport layer, the stack being intermediate between theelectrodes, wherein the organic light emitting diode satisfies thefollowing relationships:1≦μ(h1)/μ(h2)≦10⁴1≦μ(e1)/μ(e2)≦10⁴wherein μ(h1) is hole mobility of the hole transport layer; μ(e1) iselectron mobility of the electron transport layer; μ(h2) is holemobility of the light emitting layer; and μ(e2) is electron mobility ofthe light emitting layer; each of μ(h1), μ(e1), μ(h2), and μ(e2) beingfor an applied electric field of 10⁶ V/cm.

(2) An organic light emitting diode comprising a pair of electrodes anda stack including a hole transport layer, a light emitting layer, and anelectron transport layer, the stack being intermediate between theelectrodes, the light emitting layer being of a material having holemobility and electron mobility equal to or lower than hole mobility ofthe hole transport layer and electron mobility of the electron transportlayer, respectively, wherein the organic light emitting diode satisfiesthe relationship: 0.1≦μ(e2)/μ(h2)≦10.

(3) An organic light emitting diode comprising a pair of electrodes anda stack including a hole transport layer, a light emitting layer, and anelectron transport layer, the stack being intermediate between theelectrodes, the light emitting layer being of a material having holemobility and electron mobility equal to or lower than hole mobility ofthe hole transport layer and electron mobility of the electron transportlayer, respectively,

wherein the organic light emitting diode satisfies the relationship:0.1≦μ(e1)/μ(h1)≦10.

(4) The organic light emitting diode as described in any one of theabove items (1) to (3), wherein the light emitting layer is of amaterial having hole mobility and electron mobility in equilibrium.

(5) The organic light emitting diode as describe in any one of the aboveitems (1) to (4), wherein a balance of electron supply and hole supplyto the light emitting layer is in equilibrium.

(6) The organic light emitting diode as described in any one of theabove items (1) to (5), wherein the light emitting layer contains a holetransport material and an electron transport material.

(7) The organic light emitting diode as described in the above item (6),wherein the electron transport material is an aromatic heterocycliccompound having at least one hetero atom in the molecule thereof.

(8) The organic light emitting diode as describe in the above item (7),wherein the light emitting layer contains at least one of the sameelectron transport material as the electron transport layer and the samehole transport material as the hole transport layer.

(9) The organic light emitting diode as described in anyone of the aboveitems (1) to (8), wherein the light emitting layer contains aphosphorescent material.

(10) A display having a display area comprising the organic lightemitting diode as described in any one of the above items (1) to (9).

In a first aspect of the invention, an OLED comprises a pair ofelectrodes and a stack including a hole transport layer, a lightemitting layer, and an electron transport layer, the stack beingintermediate between the electrodes,

wherein the OLED satisfies the following relationships:1≦μ(h1)/μ(h2)≦10⁴  (Formula (1))1≦μ(e1)/μ(e2)≦10⁴  (Formula (2));

preferably satisfies the following relationships:1≦μ(h1)/μ(h2)≦10³1≦μ(e1)/μ(e2)≦10³;

and more preferably satisfies the following relationships:1≦μ(h1)/μ(h2)≦10^(2.5)1≦μ(e1)/μ(e2)≦10^(2.5)wherein μ(h1) is hole mobility of the hole transport layer; μ(e1) iselectron mobility of the electron transport layer; μ(h2) is holemobility of the light emitting layer; and μ(e2) is electron mobility ofthe light emitting layer; each of μ(h1), μ(e1), μ(h2), and μ(e2) beingfor an applied electric field of 10⁶ V/cm.

In a second aspect of the invention, an OLED comprises a pair ofelectrodes and a stack including a hole transport layer, a lightemitting layer, and an electron transport layer, the stack beingintermediate between the electrodes, the light emitting layer being of amaterial having hole mobility and electron mobility equal to or lowerthan hole mobility of the hole transport layer and electron mobility ofthe electron transport layer, respectively,

wherein the OLED satisfies the relationship:0.1≦μ(e2)/μ(h2)≦10  (Formula (3)),

preferably satisfies the relationship: 0.15≦μ(e2)/μ(h2)≦6;

and more preferably satisfies the relationship: 0.33≦μ(e2)/μ(h2)≦3.

In a third aspect of the invention, an OLED comprises a pair ofelectrodes and a stack including a hole transport layer, a lightemitting layer, and an electron transport layer, the stack beingintermediate between the electrodes, the light emitting layer being of amaterial having hole mobility and electron mobility equal to or lowerthan hole mobility of the hole transport layer and electron mobility ofthe electron transport layer, respectively,

wherein the OLED satisfies the relationship:0.1≦μ(e1)/μ(h1)≦10  (Formula (4));

preferably satisfies the relationship: 0.15≦μ(e1)/μ(h1)≦6;

and more preferably satisfies the relationship: 0.33≦μ(e1)/μ(h1)≦3.

By these configurations, the carrier retention time in the lightemitting layer is extended to increase the electron-hole recombinationprobability. As a result, the luminescence efficiency increases, and theelectrons and holes that pass through to the opposing transport layerswithout participating in electron-hole recombination reduce to improvethe life. The terms “hole mobility” and “electron mobility” as usedherein as to the hole transport layer, light emitting layer, andelectron transport layer mean values obtained by preparing a singlelayer (thickness: about 2 μm) of the same material as of thecorresponding layer sandwiched in between a pair of electrodes andmeasuring the hole or electron mobility of the single layer by atime-of-flight technique.

In the first aspect of the invention, both of the formula (1) and theformula (2) are satisfied. In the second aspect of the invention, theformula (3) is satisfied. In the second aspect, it is preferable that atleast one of the formulae (1), (2) and (4) is also satisfied, and it ismore preferable that all of the formulae (1), (2) and (4) are alsosatisfied. In the third aspect of the invention, the formula (4) issatisfied. In the third aspect, it is preferable that at least one ofthe formulae (1), (2) and (3) is also satisfied, and it is morepreferable that all of the formulae (1), (2) and (3) are also satisfied.

In a preferred embodiment of the OLED of the invention, the lightemitting layer is made of a material having the hole mobility and theelectron mobility in equilibrium. By this configuration, theelectron-hole recombination probability further increases to bring aboutincreased luminescence efficiency and extended life.

In another preferred embodiment of the OLED of the invention, thebalance of electron supply and hole supply to the light emitting layeris in equilibrium. By this configuration, the electron-holerecombination probability further increases to bring about furtherimproved luminescence efficiency and extended life.

The invention also provides other preferred embodiments of the OLED, inwhich:

the light emitting layer contains a hole transport material and anelectron transport material;

the electron transport material in the light emitting layer is anaromatic heterocyclic compound having at least one hetero atom in themolecule thereof;

the light emitting layer and the electron transport layer contain thesame electron transport material;

the light emitting layer and the hole transport layer contain the samehole transport material; and/or

the light emitting layer further contains a phosphorescent material.

The invention also provides still another preferred embodiment, in whichthe OLED satisfies the following relationships:μ(h2)≦μ(h1)μ(e2)≦μ(e1)wherein μ(h1) is hole mobility of the hole transport layer; μ(e1) iselectron mobility of the electron transport layer; μ(h2) is holemobility of the light emitting layer; and μ(e2) is electron mobility ofthe light emitting layer; each of which is for an applied electric fieldof 10⁶ V/cm.

In yet another preferred embodiment of the invention, the OLED satisfiesthe relationship: 0.1≦μ(e2)/μ(h2)≦10 and/or the relationship:0.1≦μ(e1)/μ(h1)≦10.

The invention also provides, in its second aspect, a display having adisplay area composed of the OLED according to the invention. Thedisplay with this configuration exhibits sufficient luminescenceefficiency and life withstanding practical use.

The present invention provides an OLED having a high external quantumefficiency and extended service life and a display using the OLED.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a layer structure of an OLED according to anembodiment of the present invention.

FIG. 2 illustrates hole mobility vs. electric field plots of the holetransport layer and the light emitting layer of an OLED according to anembodiment of the present invention. The values on the right hand sideof the right ordinate are the hole mobilities under an applied electricfield of 10⁶ V/cm. The symbol E alongside the ordinate and abscissadenotes an exponent of 10 (5.0E+02, for example, means 5.0×10²).

DETAILED DESCRIPTION OF THE INVENTION

The light emitting diode of the present invention has, on a substrate, acathode, an anode, and a stack of organic compound layers between thecathode and the anode. The stack of organic compound layers includes ahole transport layer, a light emitting layer, and an electron transportlayer. The stack may further have an additional organic compound layerbetween the electrode and the organic compound layer adjacent to thelight emitting layer. In nature of a light emitting diode, at least oneof the anode and the cathode is preferably transparent. The anode istransparent in an ordinary configuration.

The organic compound layers stack may have a charge blocking layerbetween the hole transport layer and the light emitting layer or betweenthe light emitting layer and the electron transport layer.

The organic compound layers stack preferably has a structure of holeinjection layer, hole transport layer, light emitting layer, holeblocking layer, electron transport layer, and electron injection layerin the order described from the anode side.

Where the stack has a hole blocking layer between the light emittinglayer and the electron transport layer, the organic compound layersadjacent to the light emitting layer are the hole transport layer on theanode side and the hole blocking layer on the cathode side. A holeinjection layer may be provided between the anode and the hole transportlayer. An electron injection layer may be provided between the cathodeand the electron transport layer. Each of the recited organic compoundlayers may be composed of two or more sublayers.

Each of the organic compound layers can be formed by any convenientprocesses including dry film formation techniques, such as vacuumevaporation and sputtering, a transfer process, and printing.

The light emitting layer functions to receive positive holes from thehole transport layer and electrons from the electron transport layer andallow the holes and electrons to recombine to emit light. The lightemitting layer used in the invention should be made of a material whosehole mobility and electron mobility are equal to or lower than the holemobility of the hole transport layer and the electron mobility of theelectron transport layer, respectively.

The material making the light emitting layer is preferably a materialhaving the hole and electron mobilities in equilibrium.

To satisfy the above-described conditions, the light emitting layerpreferably contains a host material and a phosphorescent material as adopant. The host material is preferably a charge transport material (theterm “charge transport” is intended to mean electron transport and holetransport inclusively). Still preferably, the light emitting layercontains both a hole transport material and an electron transportmaterial. It is preferred that at least one of the hole transportmaterial and the electron transport material of the light emitting layerbe the same as the charge transport material used in the hole transportlayer or the electron transport layer.

The host material that can be used in the light emitting layer includescompounds having a pyrene skeleton, compounds having a carbazoleskeleton, compounds having a diarylamine skeleton, compound having apyridine skeleton, compounds having a pyrazine skeleton, compoundshaving a triazine skeleton, and compounds having an arylsilane skeleton.

It is preferred that the lowest excited multiplet energy level T1 of thehost material be higher than that of the dopant material. The lightemitting layer made of the host material doped with the dopant materialis conveniently formed by vacuum co-deposition of the host material andthe dopant material.

The phosphorescent material that can be incorporated into the lightemitting layer is, in general, preferably a complex containing atransition metal atom or a lanthanoid atom. Either one or more than onephosphorescent material can be used.

Examples of the transition metal atom include, but are not limited to,ruthenium, rhodium, palladium, tungsten, rhenium, osmium, iridium, andplatinum, with rhenium, iridium, and platinum being preferred. Examplesof the lanthanoid atom include, but are not limited to, lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.Preferred of the lanthanoid atoms are neodymium, europium, andgadolinium.

The ligands of the complex include those described in G. Wilkinson etal., Comprehensive Coordination Chemistry, Pergamon Press (1987), H.Yersin, Photochemistry and Photophysics of Coordination Compounds,Springer-Verlag (1987), and Yamamoto Akio, Yuki kinzoku kagakukiso-to-ohyo, Shokabo (1982). Specific examples of suitable ligandsinclude halogen ligands (preferably a chlorine ligand),nitrogen-containing heterocyclic ligands (e.g., phenylpyridine,benzoquinoline, quinolinol, bipyridyl, and phenanthroline), diketoneligands (e.g., acetylacetone), carboxylic acid ligands (e.g., anaceticacid ligand), a carbon monoxide ligand, an isonitrile ligand, and acyano ligand, with nitrogen-containing heterocyclic ones beingparticularly preferred. The complex may be either a mononuclear onehaving one center metal atom or a bi- or polynuclear one having two ormore center metal atoms which may be the same or different.

The phosphorescent material is preferably present in an amount of 0.1%to 20% by weight, still preferably 0.5% to 10% by weight, in the lightemitting layer.

As stated, the light emitting layer may be composed of two or moresublayers which may have different luminescent colors.

If desired, the light emitting layer may contain an electrically inertbinder resin in addition to the above-described components.

The thickness of the light emitting layer is not critical but is, ingeneral, preferably 1 to 500 nm, still preferably 5 to 200 nm, evenstill preferably 10 to 100 nm.

The hole injection layer and the hole transport layer function toreceive holes from the anode or anode side and transport them toward thecathode side. The hole injection layer and the hole transport layerpreferably contain, as a hole transport material, a compound selectedfrom carbazole derivatives, triazole derivatives, oxazole derivatives,oxadiazole derivatives, imidazole derivatives, polyarylalkanederivatives, pyrazoline derivatives, pyrazolone derivatives,phenylenediamine derivatives, arylamine derivatives, amino-substitutedchalcone derivatives, styrylanthracene derivatives, fluorenonederivatives, hydrazone derivatives, stilbene derivatives, silazanederivatives, aromatic tertiary amine compounds, styrylamine compounds,aromatic dimethylidyne compounds, porphyrin compounds, organosilanederivatives, and carbon.

The thickness of the hole injection layer and the hole transport layeris preferably not more than 50 nm from the viewpoint of driving voltagereduction. More specifically, the thickness of the hole transport layeris preferably 5 to 50 nm, still preferably 10 to 40 nm, and that of thehole injection layer is preferably 0.5 to 50 nm, still preferably 1 to40 nm.

The hole transport materials may be used either individually or as amixture of two or more thereof. The hole injection layer and the holetransport layer may be each composed of two or more sublayers which maybe the same or different in composition.

The electron injection layer and the electron transport layer functionto receive electrons from the cathode or the cathode side and transportthem toward the anode side. The electron injection layer and theelectron transport layer preferably contain, as an electron transportmaterial, a compound selected from triazole derivatives, oxazolederivatives, oxadiazole derivatives, imidazole derivatives, fluorenonederivatives, anthraquinodimethane derivatives, anthrone derivatives,diphenylquinone derivatives, thiopyran dioxide derivatives, carbodiimidederivatives, fluorenylidenemethane derivatives, distyrylpyrazinederivatives; aromatic (e.g., naphthalene or perylene) tetracarboxylicacid anhydrides; phthalocyanine derivatives; various metal complexes,such as metal complexes of 8-quinolinol derivatives,metallo-phthalocyanines, and metal complexes having benzoxazole orbenzothiazole as a ligand; or organosilanes. An electron transport layercontaining an aromatic heterocyclic compound having at least one heteroatom in the molecule thereof as an electron transport material isparticularly preferred. The aromatic heterocyclic compound is a heterocompound with aromaticity, including pyridine, pyrazine, pyrimidine,pyridazine, triazine, pyrazole, imidazole, benzimidazole, triazole,thiazole, benzothiazole, isothiazole, benzisothiazole, thiadiazole, andfused rings thereof.

The thickness of the electron injection layer and the electron transportlayer is preferably not more than 50 nm from the viewpoint of drivingvoltage reduction. More specifically, the thickness of the electrontransport layer is preferably 5 to 50 nm, still preferably 10 to 50 nm,and that of the electron injection layer is preferably 0.1 to 50 nm,still preferably 0.5 to 20 nm.

The electron transport materials may be used either individually or as amixture of two or more thereof. The electron injection layer and theelectron transport layer may be each composed of two or more sublayerswhich may be the same or different in composition.

Since the electron transport layer is adjacent to the light emittinglayer, it is preferably made of a material with an ionization potentialof 6.0 eV or less to secure extended life.

The hole blocking layer functions to prevent the holes transported fromthe anode side to the light emitting layer from escaping to the cathodeside. The hole blocking layer preferably contains an aluminum complex,e.g., BAlq, a triazole derivative or a pyrazabole derivative. Where thehole blocking layer is provided as an organic compound layer adjacent tothe light emitting layer, it is usually made of a material with anionization potential of 6.0 eV or less to secure life extension.

The thickness of the hole blocking layer is preferably not more than 50nm from the viewpoint of driving voltage reduction, still preferably 1to 50 nm, even still preferably 5 to 40 nm.

The whole OLED may be protected with a protective layer. The protectivelayer can be of any material that prevents substances which mayaccelerate deterioration of the diode, such as moisture and oxygen, fromentering the diode. Such materials include metals, e.g., In, Sn, Pb, Au,Cu, Ag, Al, Ti, and Ni; metal oxides, e.g., MgO, SiO, SiO₂, Al₂O₃, GeO,NiO, CaO, BaO, Fe₂O₃, Y₂O₃, and TiO₂; metal nitrides, e.g., SiN_(x) andSiN_(x)O_(y); metal fluorides, e.g., MgF₂, LiF, AlF₃, and CaF₂;polyethylene, polypropylene, polymethyl methacrylate, polyimide,polyurea, polytetrafluoroethylene, polychlorotrifluoroethylene,polydichlorodifluoroethylene,chlorotrifluoroethylene-dichlorodifluoroethylene copolymers,tetrafluoroethylene copolymers, fluorine-containing copolymers having acyclic structure in the main chain thereof; water absorbing substanceshaving a water absorption of at least 1%; and moisture-proof substanceshaving a water absorption of 0.1% or less.

Methods for forming the protective layer include, but are not limitedto, vacuum evaporation, sputtering, reactive sputtering, molecular beamepitaxy, cluster ion beam-assisted deposition, ion plating, plasmapolymerization (radiofrequency-excited ion plating), plasma-enhancedCVD, laser-assisted CVD, thermal CVD, gas source CVD, wet coatingtechniques, printing, and transfer.

The whole OLED may be sealed in a sealing container. A desiccant or aninert liquid may be put in a space between the sealing container and theOLED. Useful desiccants include, but are not limited to, barium oxide,sodium oxide, potassium oxide, calcium oxide, sodium sulfate, calciumsulfate, magnesium sulfate, phosphorus pentoxide, calcium chloride,magnesium chloride, copper chloride, cesium fluoride, niobium fluoride,calcium bromide, vanadium bromide, molecular sieve, zeolite, andmagnesium oxide. Useful inert liquids include, but are not limited to,paraffins, liquid paraffins, fluorine-containing solvents (e.g.,perfluoroalkanes, perfluoroamines, and perfluoroethers),chlorine-containing solvents, and silicone oils.

The OLED emits light on applying direct current electricity (which maycontain an alternating component, if needed) between the anode and thecathode usually at a voltage of 2 to 15 V and a current density of 1 to100 mA/cm². For driving the OLED, the methods taught in JP-A-2-148687,JP-A-6-301355, JP-A-5-29080, JP-A-7-134558, JP-A-8-234685,JP-A-8-241047, Japanese Patent 2784615, and U.S. Pat. Nos. 5,828,429 and6,023,308 can be utilized.

An embodiment of the present invention will be illustrated by way ofFIG. 1. FIG. 1 represents a layer structure of an OLED 1 according tothe embodiment, which is used to constitute the display area of adisplay. The OLED 1 includes a substrate 2, an anode 3 on the substrate2, a hole injection layer 4 on the anode 3, a hole transport layer 5 onthe hole injection layer 4, a light emitting layer 6 on the holetransport layer 5, an electron transport layer 7 on the light emittinglayer 6, and an electron injection layer/cathode 8 on the electrontransport layer 7.

The substrate 2 that can be used in the embodiment is preferably onethat does not scatter nor attenuate light emitted from the lightemitting layer 6. Materials providing such substrates include inorganicmaterials, such as yttrium-stabilized zirconia (YSZ) and glass; andorganic materials, such as polyesters (e.g., polyethylene terephthalate,polybutylene phthalate, and polyethylene naphthalate), polystyrene,polycarbonate, polyether sulfone, polyarylate, polyimide,polycycloolefins, norbornene resins, and poly(chlorotrifluoroethylene).

When a glass substrate is used, alkali-free glass is preferred foravoiding leaching of ions from glass. In using soda lime glass, onehaving a barrier coat of silica, etc. is preferred. The organic materialas a substrate is preferably excellent in heat resistance, dimensionalstability, solvent resistance, electrical insulation, andprocessability.

The shape, structure, and size of the substrate are not particularlylimited and selected as appropriate for the intended use or purpose ofthe light emitting diode. In general, the substrate has a plate shapeand may have either a single layer structure or a multilayer structure.It may be made of a single member or two or more members. The substratemay be either colorless transparent or colored transparent. A colorlesstransparent substrate is preferred for avoiding light scattering orattenuation.

A moisture proof layer (gas barrier layer) may be provided on one orboth sides of the substrate. The moisture proof layer is preferably madeof an inorganic substance, such as silicon nitride or silicon oxide. Themoisture proof layer of such material can be formed by high frequencysputtering or like techniques. When a thermoplastic substrate is used, ahard coat or an undercoat may be provided thereon according tonecessity.

The anode 3 used in the embodiment usually serves as an electrodesupplying positive holes to the light emitting layer 4. The shape,structure, size, and the like are selected appropriately according tothe use of the light emitting diode from among known electrodematerials. As previously stated, the anode is usually formed as atransparent electrode.

Materials making up the anode include metals, alloys, metal oxides,electrically conductive compounds, and mixtures thereof. Those having awork function of 4.0 eV or higher are preferred for use as an anode.Examples of useful anode materials are conductive metal oxides, such astin oxide doped with antimony or fluorine (ATO or FTO), tin oxide, zincoxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide(IZO); metals, such as gold, silver, chromium, and nickel; mixtures orlaminates of these metals and conductive metal oxides; electricallyconductive inorganic substances, such as copper iodide and coppersulfide; electrically conductive organic substances, such aspolyaniline, polythiophene, and polypyrrole; and laminates of thesematerials and ITO. Preferred of them are electrically conductive metaloxides. ITO is particularly preferred from the viewpoint ofproductivity, high electrical conductivity, and transparency.

The anode 3 can be formed on the substrate 2 by an appropriate techniqueselected according to the material from, for example, wet film formationprocesses including printing and wet coating, physical processesincluding vacuum deposition, sputtering, and ion plating, and chemicalprocesses including CVD and plasma-enhanced CVD. For instance, an ITOanode is formed by direct current or radio frequency sputtering, vacuumdeposition or ion plating.

The position where the anode is formed is not particularly limited anddetermined according to the intended use and purpose of the lightemitting diode. The anode is, in general, preferably formed on thesubstrate 2 as in the embodiment shown in FIG. 1. In this case, theanode may be provided on either a part of or the entire area of one sideof the substrate 2. In the former case, patterning of the anode 3 iscarried out by chemical etching by photolithography, physical etching bylaser machining, vacuum deposition or sputtering through a mask, alift-off process or printing.

While the thickness of the anode 3 is not generally specified because itshould be decided as appropriate to the material, it usually ranges fromabout 10 nm to about 50 μm, preferably 50 nm to 20 μm.

The anode preferably has a surface resistivity of 10³ Ω/square or less,still preferably 10² Ω/square or less. Where the anode should be atransparent electrode, it may be either colorless or colored. To attainhigh light extraction efficiency from the transparent anode side, it ispreferred for the anode to have a transmittance of at least 60%, stillpreferably 70% or higher.

Details of a transparent anode are described in Sawada Yutaka (ed.),Tomei Denkyokumaku no Shin-tenkai, CMC (1999), which also apply to theOLED of the present embodiment. Where a plastic substrate with low heatresistance is used, an ITO or IZO transparent anode formed at or below150° C. is recommended.

The cathode that can be used in the present embodiment usually functionsto supply electrons to the electron injection layer. The shape,structure, size, etc. of the cathode are not particularly limited andselected from among known electrode materials according to the use ofthe light emitting diode.

Materials making up the cathode include metals, alloys, metal oxides,electrically conductive compounds, and mixtures thereof. Those having awork function of 4.5 eV or less are preferred. Examples of usefulcathode materials are alkali metals (e.g., Li, Na, K, and Cs), alkalineearth metals (e.g., Mg and Ca), gold, silver, lead, aluminum,sodium-potassium alloys, lithium-aluminum alloys, magnesium-silveralloys, and rare earth metals (e.g., indium and ytterbium). Whileeffective even when used individually, these materials are preferablyused as a combination of two or more thereof to assure both stabilityand electron injection capabilities.

Of the recited cathode materials preferred are alkali metals andalkaline earth metals for their electron injection capabilities, andaluminum-based materials are also preferred for their storage stability.The term “aluminum-based materials” includes aluminum and an alloy ormixture of aluminum with 0.01% to 10% by weight of an alkali metal or analkaline earth metal, e.g., a lithium-aluminum alloy or amagnesium-aluminum alloy. For the details of cathode materials referencecan be made to JP-A-2-15595 and JP-A-5-121172.

The cathode can be formed by any known method selected as appropriate tothe material from, for example, wet film formation by printing orcoating; physical film formation including vacuum deposition,sputtering, and ion plating; and chemical film formation including CVDand plasma-enhanced CVD. For example, a metal or like cathode can beformed by sputtering a metallic material or materials. In using two ormore materials, they may be sputtered either simultaneously orsequentially.

Patterning of the cathode is carried out by chemical etching byphotolithography, physical etching by laser machining, vacuum depositionor sputtering through a mask, a lift-off process or a printing process.

The position of forming the cathode in the light emitting diode of thepresent embodiment is not particularly limited. It is formed on eitherthe entire area or a part of the electron injection layer. A dielectriclayer of, for example, a fluoride or an oxide of an alkali metal or analkaline earth metal can be formed between the cathode and the electroninjection layer to a thickness of 0.1 to 5 nm. Such a dielectric layermay be regarded as a kind of an electron injection layer. The dielectriclayer can be formed by vacuum evaporation, sputtering, ion plating, etc.In FIG. 1, the cathode and the electron injection layer are depicted asa monolithic layer indicated by numeral 8.

The thickness of the cathode is decided as appropriate for the materialand usually ranges from about 10 nm to about 5 μm, preferably 50 nm to 1μm. The cathode may be either transparent or opaque. The transparentcathode can be formed by forming a film as thin as 1 to 10 nm of theabove recited material and stacking thereon a transparent conductivematerial such as ITO or IZO.

The OLED according to the embodiment of FIG. 1 has a stack of organiccompound layers including the light emitting layer 6. Organic compoundlayers other than the light emitting layer 6 include the hole transportlayer 5, the electron transport layer 7, the hole injection layer 4, andthe electron injection layer 8.

The OLED 1 of the embodiment has an ITO layer as the anode 3, a copperphthalocyanine (CuPc) layer with a thickness of about 10 nm as the holeinjection layer 4, a compound A (shown below) layer with a thickness ofabout 50 nm as the hole transport layer 5, a co-deposited layer ofcompound A and Alq (shown below) at a ratio of 50:50 with a thickness ofabout 40 nm as the light emitting layer 6, and an Alq layer with athickness of about 20 nm as the electron transport layer. LiF is used asan electron transport layer, and Al as a cathode material.

The organic compound layers used in the OLED 1 can each be convenientlyformed by any of dry film formation processes including vacuumevaporation and sputtering, transfer, printing, and the like.

As previously mentioned, the hole transport layer 5, the electrontransport layer 7, and the light emitting layer 6 of the OLED 1 are acompound A layer, an Alq layer, and a 50:50 co-deposited layer ofcompound A and Alq, respectively. In this connection, FIG. 2 graphicallyrepresents the hole mobilities of compound A and compound A/Alqmixtures. It is seen from FIG. 2 that the compound A/Alq mixtures havelower hole mobilities than compound A and that the 50:50 mixture ofcompound A/Alq has lower hole mobilities than the 75:25 mixture ofcompound A/Alq.

Generation of light energy in an OLED is generally based onhole-electron recombination in the light emitting layer 6 between theholes injected from the hole transport layer 5 to the light emittinglayer 6 and the electrons injected from the electron transport layer 7to the light emitting layer 6. The materials making the layers 5, 6, and7 have hitherto been selected so as to form a potential barrier betweenthe light emitting layer 6 and the hole transport layer 5 and betweenthe light emitting layer 6 and the electron transport layer 7 thereby toprevent the injected holes or electrons from escaping outside from thelight emitting layer 6.

In contrast, the OLED 1 of the present embodiment is characterized inthat the materials of the light emitting layer 6, the electron transportlayer 7, and the hole transport layer 5 are designed not to positivelyheighten the potential barrier but such that the light emitting layer 6may have a smaller hole mobility and electron mobility than the holemobility of the hole transport layer 5 and the electron mobility of theelectron transport layer 7, respectively. This material design aims tohave the electrons and holes injected into the light emitting layer 6remain there long to increase the recombination probability thereby toachieve increased emission efficiency.

Holes not having been recombined with electrons in the light emittinglayer 6 enter the electron transport layer 7 and damage the layer 7.Likewise, electrons not having been recombined with holes in the lightemitting layer 6 enter the hole transport layer 5 to damage the layer 5.Since the OLED 1 has an increased hole-electron recombinationprobability in the light emitting layer 6, it naturally follows that theprobability of the damage to the electron transport layer 7 and the holetransport layer 5 decreases thereby to increase the service life of thelight emitting diode.

More specifically, the OLED 1 of the present embodiment has the holetransport layer 5, the light emitting layer 6, and the electrontransport layer 7 provided between a pair of the electrodes 3 and 8using the respective materials that are preferably selected to satisfythe following relationships:1≦μ(h1)/μ(h2)≦10⁴1≦μ(e1)/μ(e2)≦10⁴wherein μ(h1) is hole mobility of the hole transport layer; μ(e1) iselectron mobility of the electron transport layer; μ(h2) is holemobility of the light emitting layer; and μ(e2) is electron mobility ofthe light emitting layer; each of which is for an applied electric fieldof 10⁶ V/cm.

Or, the hole mobility μ(h2) and the electron mobility μ(e2) in the lightemitting layer 6 are in equilibrium to further increase theelectron-hole recombination probability in the light emitting layer 6.More specifically, a material providing a light emitting layersatisfying the relationship: 0.1≦μ(e2)/μ(h2)≦10 can be used.

Or, the electron mobility μ(e1) of the electron transport layer 7, whichinjects electrons into the light emitting layer 6, and the hole mobilityμ(h1) of the hole transport layer 5, which injects holes into the lightemitting layer 6, are in equilibrium to further improve luminescenceefficiency and life. More specifically, the layers 7 and 5 can be madeof materials satisfying the relationship: 0.1≦μ(e1)/μ(h1)≦10.

According to the above-described material design, the hole transportlayer 5, the electron transport layer 7, and the light emitting layer 6of the OLED 1 of the present embodiment are made of compound A, Alq, anda 50:50 mixture of compound A and Alq, respectively. This materialdesign is an illustrative example, and any other materials can be usedas long as the above relationships are fulfilled.

EXAMPLES

The present invention will now be illustrated in greater detail withreference to Examples, but it should be understood that the invention isnot construed as being limited thereto.

Example 1

A 0.2 μm thick ITO film was formed as a transparent anode on a 0.5 mmthick, 2.5 cm-side square glass plate using a DC magnetron sputtersystem under the following conditions. The ITO film thus formed had asurface resistivity of 10 Ω/square.

Target: ITO target having an indium:tin molar ratio of 95:5 and an SnO₂content of 10 wt %.

Substrate temperature: 250° C.

Oxygen pressure: 1×10⁻³ Pa

The substrate with the transparent anode was put into a washingcontainer and washed with isopropyl alcohol and then treated withUV/ozone for 30 minutes.

Copper phthalocyanine (CuPc) was deposited on the ITO anode by vacuumevaporation at a deposition rate of 1 nm/sec to form a hole injectionlayer with a thickness of 0.01 μm. Compound A was deposited thereon byvacuum evaporation at a rate of 1 nm/sec to form a hole transport layerwith a thickness of 0.03 μm.

Compound A and Alq were co-deposited on the hole transport layer byvacuum evaporation at a mixing weight ratio of 50:50 to form a 0.03 μmthick light emitting layer.

Alq was deposited on the light emitting layer by vacuum evaporation at arate of 1 nm/sec to form an electron transport layer with a depositthickness of 0.05 μm.

A patterning mask providing a luminescent area of 2 mm by 2 mm wasplaced on the electron transport layer, and LiF was deposited at a rateof 1 nm/sec by vacuum evaporation to form an electron injection layerhaving a thickness of 0.002 μm.

With the patterning mask remaining on the electron transport layer,aluminum was deposited by vacuum evaporation to a thickness of 0.25 μmto form a back electrode (cathode).

An aluminum lead wire was connected to the transparent anode and thealuminum cathode to make a luminescent structure.

The following operation was carried out in a glove box purged withnitrogen. Ten milligrams of calcium oxide powder as a desiccant was putinto the recess of a stainless steel sealing cover and fixed there withadhesive tape. The luminescent structure was sealed using the thusprepared sealing cover and a UV curing adhesive (XNR5516HV from NagaseChemtex Corp.) to complete an OLED.

The resulting OLED was evaluated as follows.

(1) A DC voltage was applied to the OLED by use of Source-Measure UnitModel 2400 supplied by Toyo Corp. to measure initial luminescentperformance of the diode. The luminescence efficiency at 2000 cd/m² interms of external quantum efficiency (η₂₀₀₀) and voltage (V₂₀₀₀) isshown in Table 1 below.(2) Half decay time T_(1/2) from the initial luminance 2000 cd/m² inconstant current life testing is shown in Table 1.(3) A structure having a single layer (thickness: about 2 μm) of thesame material as each of the hole transport layer, light emitting layer,and electron transport layer sandwiched in between a pair of electrodeswas prepared, and the hole mobility and/or the electron mobility of eachstructure in an electric field of 10⁶ V was measured by a time-of-flighttechnique. The results obtained are shown below.

μ(h1) of hole transport layer (compound A): 6.5 × 10⁻⁵ cm²/V · sec μ(h2)of light emitting layer: 5.1 × 10⁻⁶ cm²/V · sec μ(e2) of light emittinglayer: 3.4 × 10⁻⁶ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

It is seen that the OLED of Example 1 satisfies the relationships:1≦μ(h1)/μ(h2)≦10⁴ and 1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratioμ(e2)/μ(h2) of 0.67 and the ratio μ(e1)/μ(h1) of 0.65, proving that theOLED also satisfies both the relationships: 0.1≦μ(e2)/μ(h2)≦10 and0.1≦μ(e1)/μ(h1)≦10.

Example 2

An OLED was fabricated and evaluated in the same manner as in Example 1,except for replacing compound A making the hole transport layer withN,N′-dinaphthyl-N,N′-diphenylbenzidine (NPD) and replacing the CompoundA/Alq mixture making the light emitting layer with Alq alone.

The results of evaluation on the resulting OLED are shown in Table 1.The hole mobility and electron mobility of each layer were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 3.2 × 10⁻⁷ cm²/V · sec μ(e2) of light emittinglayer: 4.2 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

It is seen that the OLED of Example 2 satisfies the relationships:1≦μ(h1)/μ(h2)≦10⁴ and 1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratioμ(e2)/μ(h2) of 131 and the ratio μ(e1)/μ(h1) of 0.016, proving that theOLED does not satisfy any of the relationships: 0.1≦μ(e2)/μ(h2)≦10 and0.1≦μ(e1)/μ(h1)≦10.

Example 3

An OLED was fabricated and evaluated in the same manner as in Example 2,except for replacing Alq making the light emitting layer with a 50/50mixture of Alq and NPD. The results of evaluation are shown in Table 1.The hole mobility and electron mobility of each layer were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 7.1 × 10⁻⁴ cm²/V · sec μ(e2) of light emittinglayer: 2.1 × 10⁻⁶ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

The OLED of Example 3 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴ and1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of 0.003and the ratio μ(e1)/μ(h1) of 0.016, revealing that the OLED does notsatisfy any of the relationships: 0.1≦μ(e2)/μ(h2)≦10 and0.1≦μ(e1)/μ(h1)≦10.

TABLE 1 μ₂₀₀₀ (%) V₂₀₀₀ (V) T_(1/2) (hr) Example 1 2.0 9.5 800 Example 21.1 9.0 350 Example 3 0.9 9.0 450

Example 4

A 0.2 μm thick ITO film was formed as a transparent anode on a 0.5 mmthick, 2.5 cm-side square glass plate using a DC magnetron sputtersystem under the following conditions. The ITO film thus formed had asurface resistivity of 10 Ω/square.

Target: ITO target having an indium:tin molar ratio of 95:5 and an SnO₂content of 10 wt %.

Substrate temperature: 250° C.

Oxygen pressure: 1×10⁻³ Pa

The substrate with the transparent anode was put into a washingcontainer and washed with isopropyl alcohol and then treated withUV/ozone for 30 minutes.

Copper phthalocyanine was deposited on the ITO anode by vacuumevaporation at a deposition rate of 1 nm/sec to form a hole injectionlayer with a thickness of 0.01 μm. NPD was deposited thereon by vacuumevaporation at a rate of 1 nm/sec to form a hole transport layer with athickness of 0.03 μm.

Firpic (shown below) as a phosphorescent compound, mCP (shown below) asa hole transport material, and compound B (shown below) as an electrontransport material were co-deposited on the hole transport layer byvacuum evaporation at a mixing ratio of 5/75/25 to form a 0.03 μm thicklight emitting layer.

Balq2 (shown below) was deposited on the light emitting layer at a rateof 1 nm/sec to form a blocking layer with a thickness of 0.01 μm.

Alq was deposited on the blocking layer by vacuum evaporation at a rateof 0.2 nm/sec to form an electron transport layer with a thickness of0.04 μm.

A patterning mask providing a luminescent area of 2 mm by 2 mm wasplaced on the electron transport layer, and LiF was deposited at a rateof 1 nm/sec by vacuum evaporation to form an electron injection layerhaving a thickness of 0.002 μm.

With the patterning mask remaining on the electron transport layer,aluminum was deposited by vacuum evaporation to a thickness of 0.25 μmto form a back electrode (cathode).

An aluminum lead wire was connected to the transparent anode and thealuminum cathode to fabricate a luminescent structure.

The following operation was carried out in a glove box purged withnitrogen. Ten milligrams of calcium oxide powder as a desiccant was putinto the recess of a stainless steel sealing cover and fixed there withadhesive tape. The luminescent structure was sealed using the thusprepared sealing cover and a UV curing adhesive (XNR5516HV from NagaseChemtex Corp.) to complete an OLED.

The resulting OLED was evaluated as follows.

(1) A DC voltage was applied to the OLED by use of Source-Measure UnitModel 2400 supplied by Toyo Corp. to evaluate initial luminescentperformance of the diode. The luminescence efficiency at 360 cd/m² interms of external quantum efficiency (η₃₆₀) and voltage (V₃₆₀) is shownin Table 2 below.(2) Half decay time T_(1/2) from the initial luminance 360 cd/m² inconstant current life testing is shown in Table 2.(3) The hole mobility and/or electron mobility of the hole transportlayer, light emitting layer, and electron transport layer were measuredin the same manner as in Example 1 to give the following results.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 7.3 × 10⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 6.1 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

The OLED of Example 4 satisfies the relationship: μ(h2)≦μ(h1). Themeasurements give the ratio μ(e2)/μ(h2) of 0.84, proving that the OLEDalso satisfies the relationship: 0.1≦μ(e2)/μ(h2)≦10. Thus, the formula(3) is satisfied and long life is had.

Example 5

An OLED was fabricated and evaluated in the same manner as in Example 4,except for replacing Alq as an electron transport material with compoundB. The results of evaluation are shown in Table 2. The hole mobility andelectron mobility of each layer as measured in the same manner as inExample 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 7.3 × 10⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 6.1 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer(compound B): 2.5 × 10⁻³ cm²/V · sec

The OLED of Example 5 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴ and1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of 0.84and the ratio μ(e1)/μ(h1) of 0.93, proving that the OLED also satisfiesboth the relationships: 0.1≦μ(e2)/μ(h2)≦10 and 0.1≦μ(e1)/μ(h1)≦10. Thus,the formulae (1) to (4) are satisfied, and high luminescence efficiencyand long life are both had.

Example 6

An OLED was fabricated and evaluated in the same manner as in Example 4,except for replacing the co-deposited Firpic/mCP/compound (B) as a lightemitting layer with a co-deposited layer of Firpic and mCP (depositionratio is 5:100). The results of evaluation are shown in Table 2. Thehole mobility and electron mobility of each layer as measured in thesame manner as in Example 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 9.5 × 10⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 2.8 × 10⁻⁶ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

The OLED of Example 6 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴ and1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of 0.029and the ratio μ(e1)/μ(h1) of 0.016, indicating that the OLED does notsatisfy either of the relationships: 0.1≦μ(e2)/μ(h2)≦10 and0.1≦μ(e1)/μ(h1)≦10.

TABLE 2 μ₃₆₀ (%) V₃₆₀ (V) T_(1/2) (hr) Example 4 5.8 10.0 120 Example 58.1 10.0 120 Example 6 4.2 9.0 70

Example 7

An OLED was fabricated in the same manner as in Example 4, except forreplacing the co-deposited Firpic/mCP/compound (B) as a light emittinglayer with a co-deposited layer of Ir(ppy)3 (shown below) as aphosphorescent compound, CBP (shown below) as a hole transport material,and compound C (shown below) as an electron transport material at aratio of 5:60:40. The resulting OLED was evaluated in the same manner asin Example 1. The results of evaluation are shown in Table 3 below. Thehole mobility and electron mobility of each layer as measured in thesame manner as in Example 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 1.2 × 10⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 3.1 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

The OLED of Example 7 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴ and1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of 2.6,indicating that the OLED satisfies the relationship: 0.1≦μ(e2)/μ(h2)≦10.

Example 8

An OLED was fabricated in the same manner as in Example 4, except forreplacing Alq as an electron transport material with compound C. Theresults of evaluation are shown in Table 3. The hole mobility andelectron mobility of each layer as measured in the same manner as inExample 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 7.3 × 10⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 6.1 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer(compound C): 9.0 × 10⁻⁴ cm²/V · sec

The OLED of Example 8 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴ and1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of 0.84and the ratio μ(e1)/μ(h1) of 0.33, proving that the OLED also satisfiesboth the relationships: 0.1≦μ(e2)/μ(h2)≦10 and 0.1≦μ(e1)/μ(h1)≦10.

Example 9

An OLED was fabricated and evaluated in the same manner as in Example 8,except for replacing NPD as a hole transport material with compound A.The results of evaluation are shown in Table 3. The hole mobility andelectron mobility of each layer as measured in the same manner as inExample 1 were as follows.

μ(h1) of hole transport layer (compound A): 6.5 × 10⁻⁵ cm²/V · sec μ(h2)of light emitting layer: 7.3 × 1⁻⁵ cm²/V · sec μ(e2) of light emittinglayer: 6.1 × 10⁻⁵ cm²/V · sec μ(e1) of electron transport layer(compound C): 9.0 × 10⁻⁴ cm²/V · sec

The OLED of Example 9 satisfies the relationship: 1≦μ(e1)/μ(e2)≦10⁴. Themeasurements give the ratio μ(e2)/μ(h2) of 0.84 and the ratioμ(e1)/μ(h1) of 13.85, indicating that the OLED also satisfies therelationship: 0.1≦μ(e2)/μ(h2)≦10 but does not satisfy the relationship:0.1≦μ(e1)/μ(h1)≦10. The diode does not satisfy the relationship:μ(h2)≦μ(h1), either.

Example 10

An OLED was fabricated and evaluated in the same manner as in Example 8,except for replacing the co-deposited Ir(ppy)3/CBP/compound C layer as alight emitting layer with a co-deposited layer of Ir(ppy)3/CBP(deposition ratio is 5:100). The results of evaluation are shown inTable 3. The hole mobility and electron mobility of each layer asmeasured in the same manner as in Example 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 9.8 × 1⁻⁴ cm²/V · sec μ(e2) of light emittinglayer: 1.3 × 10⁻⁷ cm²/V · sec μ(e1) of electron transport layer(compound C): 9.0 × 10⁻⁴ cm²/V · sec

The OLED of Example 10 satisfies the relationships: μ(h2)≦μ(h1) andμ(e2)≦μ(e1). The measurements give the ratio μ(e2)/μ(h2) of 0.00013 andthe ratio μ(e1)/μ(h1) of 0.33. That is, the relationship:0.1≦μ(e2)/μ(h2)≦10 is not satisfied.

Example 11

An OLED was fabricated and evaluated in the same manner as in Example 7,except for replacing the co-deposited Ir(ppY)3/CBP/compound C layer as alight emitting layer with a co-deposited layer of Ir(ppy)3/CBP(deposition ratio is 5:100). The results of evaluation are shown inTable 3. The hole mobility and electron mobility of each layer asmeasured in the same manner as in Example 1 were as follows.

μ(h1) of hole transport layer (NPD): 2.7 × 10⁻³ cm²/V · sec μ(h2) oflight emitting layer: 9.8 × 10⁻⁴ cm²/V · sec μ(e2) of light emittinglayer: 1.3 × 10⁻⁷ cm²/V · sec μ(e1) of electron transport layer (Alq):4.2 × 10⁻⁵ cm²/V · sec

The OLED of Example 11 satisfies the relationships: 1≦μ(h1)/μ(h2)≦10⁴and 1≦μ(e1)/μ(e2)≦10⁴. The measurements give the ratio μ(e2)/μ(h2) of0.00013 and the ratio μ(e1)/μ(h1) of 0.016. That is, the OLED does notsatisfy either of the relationships: 0.1≦μ(e2)/μ(h2)≦10 and0.1≦μ(e1)/μ(h1)≦10.

TABLE 3 μ₂₀₀₀ (%) V₂₀₀₀ (V) T_(1/2) (hr) Example 7 8.0 9.0 1200 Example8 9.1 8.5 1300 Example 9 5.2 9.0 650 Example 10 6.5 8.5 750 Example 116.3 9.0 700

The OLED according to the present invention has high luminescenceefficiency and long life. It is therefore suitable for practical use andprovides a practical, large-area, full-color display.

This application is based on Japanese Patent application JP 2005-52911,filed Feb. 28, 2005, the entire content of which is hereby incorporatedby reference, the same as if set forth at length.

1. An organic light emitting diode comprising a pair of electrodes and astack including a hole transport layer, a light emitting layer, and anelectron transport layer, the stack being intermediate between theelectrodes, the light emitting layer being of a material having holemobility and electron mobility equal to or lower than hole mobility ofthe hole transport layer and electron mobility of the electron transportlayer, respectively, wherein the organic light emitting diode satisfiesthe relationship: 0.1≦μ(e2)/μ(h2)≦10, wherein μ(e2) is electron mobilityof the light emitting layer and μ(h2) is hole mobility of the lightemitting layer.
 2. The organic light emitting diode as claimed in claim1, wherein the light emitting layer is of a material having holemobility and electron mobility in equilibrium.
 3. The organic lightemitting diode as claimed in claim 1, wherein a balance of electronsupply and hole supply to the light emitting layer is in equilibrium. 4.The organic light emitting diode as claimed in claim 1, wherein thelight emitting layer contains a hole transport material and an electrontransport material.
 5. The organic light emitting diode as claimed inclaim 4, wherein the electron transport material is an aromaticheterocyclic compound having at least one hetero atom in the moleculethereof.
 6. The organic light emitting diode as claimed in claim 5,wherein the light emitting layer contains at least one of the sameelectron transport material as the electron transport layer and the samehole transport material as the hole transport layer.
 7. The organiclight emitting diode as claimed in claim 1, wherein the light emittinglayer contains a phosphorescent material.
 8. A display having a displayarea comprising the organic light emitting diode as claimed in claim 1.